Loop cut stops at quad
Thanks for the pointers in the comments. Turns out I screwed up during retopo.
This was one face. The edge across was not connected to the faces, i.e. splitting them.
Found the issue by selecting faces and comparing the count to what I was seeing. I suppose this is exactly what @susu meant with &quot;disconnected faces&quot;, so cheers

I solved it by deleting the face and filling in between the edges to create the two faces I expected